rustpkg: Un-ignore most of the remaining tests

r? @graydon This necessitated some cleanup to how we parse library filenames
when searching for libraries, since rustpkg may now create filenames
that contain '-' characters. Also cleaned up how rustpkg passes the
sysroot to a custom build script.

This passed a try build, except for ```rust_path_test```, which failed only on Linux. I'm not sure why, but I re-ignored it for now.